Steel Sheet Microstructure Control for Stretch Flangeability
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Solution Overview
Problem
Conventional high-strength steel sheets face challenges in achieving both excellent stretch flangeability and fatigue properties, particularly when strain distribution is considered, and often suffer from cracking during cold pressing due to uneven work hardening at the edge portions.
Innovation Solution
The steel sheet composition and microstructure are optimized by controlling the proportion of crystal grains with intragranular misorientation between 5 to 14° and the distribution of Ti-based and Nb-based carbides on ferrite grain boundaries, along with specific chemical compositions and microstructural ratios, to enhance stretch flangeability and fatigue properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the steel sheet is increased in strength, then the weight reduction and fuel efficiency are improved, but the material properties such as formability and workability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.008-0.150%, Si: 0.01-1.70%, Mn: 0.60-2.50%, Al: 0.010-0.60%, Ti: 0-0.200%, Nb: 0-0.200%, Ti+Nb: 0.015-0.200%) and microstructural parameters (ferrite area ratio: 30-95%, bainite area ratio: 5-70%, intragranular misorientation proportion: 20-100%, aspect ratio: 5 or less, carbide distribution density: 10 carbides/μm or less) to achieve a balance between strength and formability, resolving the contradiction through optimized parameter combinations
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and bainite phases with specific proportions, along with strategically distributed Ti-based and Nb-based carbides, to achieve both high strength and good formability. This composite approach allows the material to exhibit both strength and ductility simultaneously
2Strength
If the steel sheet is increased in strength, then the tensile strength is improved, but the ductility and stretch flangeability are insufficient
Solution Approach 1:
The patent applies local quality by creating a non-uniform microstructure where ferrite phases (30-95% area ratio) are distributed throughout the steel sheet, providing local ductility and formability, while bainite phases (5-70% area ratio) and carbide precipitates provide local strength, achieving both high tensile strength and sufficient ductility through spatial distribution of different phases
Solution Approach 2:
The patent controls the intragranular misorientation parameter (proportion of crystal grains with 5-14° misorientation: 20-100%) and aspect ratio (5 or less) to optimize the balance between strength and ductility, demonstrating parameter change by adjusting microstructural parameters to achieve desired mechanical properties
3Ease of manufacture
If conventional high-strength steel sheet is subjected to cold pressing, then the forming is performed, but cracking occurs from the edge portion due to uneven work hardening
Solution Approach 1:
The patent applies local quality by distributing ferrite phases (30-95% area ratio) uniformly throughout the steel sheet, which provides consistent local ductility and work hardening capability at punched end faces and edge portions, preventing cracking during cold pressing while maintaining overall formability
Solution Approach 2:
The patent applies preliminary action by controlling the microstructure before forming operations, specifically by distributing ferrite phases and controlling carbide distribution to prevent uneven work hardening that would lead to cracking during subsequent cold pressing operations
4Shape
If the steel sheet is used for complex shape parts requiring stretch flanging and burring, then the complex shape is achieved, but the stretch flangeability and ductility are required to be good
Solution Approach 1:
The patent applies local quality by creating a microstructure with ferrite phases (30-95% area ratio) distributed throughout, which provides local ductility and stretch flangeability at punched end faces and forming zones, enabling the steel sheet to be formed into complex shapes while maintaining good stretch flangeability
Data Source
AI summary
A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 30 to 95%, and bainite: 5 to 70%. When a region that is surrounded by a grain boundary having a misorientation of 15° or more and has a circle-equivalent diameter of 0.3 μm or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14° to all crystal grains is 20 to 100% by area ratio. An average aspect ratio of ellipses equivalent to the crystal grains is 5 or less. An average distribution density of the total of Ti-based carbides and Nb-based carbides each having a grain size of 20 nm or more on ferrite grain boundaries is 10 carbides/μm or less.